Granulosa cells from estrogen-primed, hypophysectomized immature rats have been transformed with SV-40 virus and established as cell lines in vitro. After preliminary passage through nude mice, these cells were demonstrated to form both ascites and solid tumors in Sprague-Dawley female rats. Culture of the ascites fluid rapidly re-established the cell line in vitro. Growth of the tumor is accelerated by bilateral oophorectomy and retarded by hypophysectomy. This system may serve as an in vivo/in vitro model for the study of chemotherapeutic agents and understanding of the pathophysiology of ovarian carcinoma.